1. Field of the Invention
The present invention relates to a process for surface-treating an aluminum-containing material, particularly a heat-exchanger having heat-exchanging tubes and fins comprising aluminum or an aluminum alloy and usable as a part of an air conditioner for motorcars.
2. Description of the Related Art
Conventional heat exchangers having heat-exchanging tubes and fins comprising aluminum or an aluminum alloy are mostly designed so that the surface areas of heat-radiating portions and cooling portions are as large as possible, to obtain excellent heat-radiation or cooling effects in a limited space. Therefore, the gaps between the fins are very small. Also, to decrease air resistance of the heat exchanger to as low as possible, the fins are notched. The notched fin is referred to as a fin louver.
When the above-mentioned heat exchangers are used for cooling, the moisture contained in air is condensed on the surface of the heat exchanger to form water drops which fill the gaps between the fins to increase the air resistance of the heat exchanger, and thus the heat-exchanging efficiency of the heat-exchanger is decreased.
Also, the condensed water drops cause corrosion of aluminum or aluminum alloy in the heat exchanger, and thus a fine white powder of aluminum oxide is generated on the fin surfaces. When the heat exchanger surface is kept in a wetted condition for long time, mold easily grows on the fin surfaces.
The white aluminum oxide powder formed on the fin surfaces and the water drops condensed between the fins are scattered by an air blower into the passenger compartment of the motorcar, and the mold grown on the fin surfaces generates mold odor, to give the occupants an unpleasant feeling.
As a surface treatment for a purpose of preventing a corrosion of the aluminum or aluminum alloy heat exchanger, a chromic acid-chromate chemical conversion treatment and a phosphoric acid-chromate chemical conversion treatment are known. The chromic acid-chromate chemical conversion treatment was practically utilized from about 1950 and is still widely used for the fin materials of heat exchangers, etc. This chemical conversion treatment liquid contains, as main components, chromic acid (CrO.sub.3) and hydrofluoric acid (HF), and further an accelerator, and can form a chemical conversion coating containing a small amount of hexavalent chromium. The phosphoric acid-chromate chemical conversion treatment is based on the invention of U.S. Pat. No. 2,438,877, and the treatment liquid thereof comprises chromic acid (CrO.sub.3), phosphoric acid (H.sub.3 PO.sub.4) and hydrofluoric acid (HF). The resultant chemical conversion coating contains, as a principal component, hydrated chromium phosphate (CrPO.sub.4.multidot.4H.sub.2 O).
To prevent the blockage of the heat exchanger by the water drops remaining in the gaps between the fins, treatment methods for imparting a high hydrophilicity to the fin surfaces and for enhancing the water-wetting property of the fin surfaces have been developed. In these methods, a hydrophilic coating is formed from hydrophilic inorganic compounds, for example, water glass and silica gel, and organic compounds, for example, surfactants and water-soluble resin, which may be used alone or in a mixture of two or more thereof, on a surface of corrosion resistant coating for example, phosphoric acid-chromate coating or chromic acid-chromate coating.
For example, Japanese Unexamined Patent Publication No. 61-250,495 discloses an aluminum heat exchanger and a process for producing the same.
This process is characterized in that a hydrophilic coating comprising, as a principal component, a water-soluble polyamide resin exhibiting a cationic property in an aqueous solution thereof is formed on a chemical conversion coating such as chromate coating. This process is, however, disadvantageous in that the coating procedure causes a waste liquid containing hexavalent chromium (Cr.sup.6+) to be discharged. Since the chromate type surface treatment uses an aqueous treatment liquid containing harmful hexavalent chromium, there is a strong demand for a new treatment liquid containing no hexavalent chromium, to prevent environmental pollution. Also, since the above-mentioned waste liquid is not allowed to be discharged without a hexavalent chromium-removing treatment, the waste liquid must be treated by a treatment apparatus using treatment reagents which causes the resultant product to be expensive.
To solve the above-mentioned problem, for example, Japanese Unexamined Patent Publication No. 7-48,682 discloses a surface treatment process for an aluminum heat exchanger comprising the steps of forming a surface treatment layer on the aluminum surface by an anti-rust agent selected from water-soluble polyaminepolyamide resin-tannic acid-titanium compositions, water-soluble polyamide resin-tannic acid-zirconium compositions, water-soluble acryl-styrene copolymer resin-phytic acid-zirconium compositions, polyvinyl alcohol-tannic acid-lithium compositions, tannic acid-titanium compositions, tannic acid, zirconium compositions, tannic acid-lithium compositions, phytic acid-lithium compositions, phytic acid zirconium compositions, phytic acid-titanium compositions and silane-coupling agents; and optionally forming a hydrophilic coating layer containing an antibacterial agent on the surface treatment layer.
By applying the above-mentioned process, the problem of the waste liquid containing the hexavalent chromium (Cr.sup.6+) can be solved, and the treatment cost including the treatment apparatus cost and the treatment reagent cost can be saved.
However, generally, the non-chromate coating exhibits a lower corrosion resistance than that of the chromate coating. Therefore, in the aluminum heat exchanger obtained in accordance with the process disclosed in the Japanese unexamined patent publication by passing through the surface treatment procedure using, as a chemical conversion coating, the non-chromate coating, when water-absorption and swelling and then drying are repeatedly applied to the heat exchanger by the drying cycle, the formed coating is deteriorated and thus, the coating is partly peeled away to create defective portions, an odor is generated due to the metal or metal oxides exposed in the defective portions, and the odor is blown into the inner room space of the motorcar by the air blower attached to the heat exchanger to give the occupants in the motorcar an unpleasant feeling. The above-mentioned disadvantages have not yet been overcome.
Accordingly, at the present time, there is no process for surface-treating an aluminum heat exchanger with a non-chromate treating liquid, so that the resultant treated surface can maintain high hydrophilicity, corrosion resistance and resistance to odor-generation over a long period, and the waste liquid-treating cost is reduced.